He's just "Crazy Superman" and Marvel can't keep his origin straight. He's also so OP that writers had to find ways to make sure he was rarely involved in battles. Granted, Bendis did the bulk of the writing for him over the years, so maybe under a different writer, he might be interesting.

I actually think you could do something with the Sentry. I like the idea that every time you do something heroic, you're evil counterpoint comes out to do something horrible. Heck, give the Void pre-cog powers so he can target specific places or people. "Oh sure, you saved the city, but I killed the kid who would have cured cancer" type deal.